I'm going to assume that what I see is what you wanted to say:
3a^3 is in the numerator, and (3a^3 + 6a^2) is in the denominator.
You have one term in the numerator and two terms in the denoninator. These terms have some factors in common: each of them has at least a 3, and each of them has at least an a^2. So we can divide out ("cancel") these factors from each term, which will leave:
a in the numerator, and (a + 2) in the denominator.
Another way to look at it is to factor the denominator into:
3a^2(a + 2)
Write the numerator as: a*3a^2
Then "cancel" the 3a^2 in the denominator with the 3a^2 in the numerator, which will leave just an "a" in the numerator, and the a+2 in the denominator.